A TROPICAL TWIST ON FRUIT EVOLUTION

by Kate Galloway

droppedImage-2In the peaceful sanctuary of Panama’s tropical trees, toucan and parrot calls pierce the hot, humid air. Flying harpy eagles streak the sky as they search for their next meal. Spider monkeys gracefully swing from the towering canopy, and jaguars prowl through the lush jungle.

Perched on this fertile ground sits Ingrid Parker, a plant biologist at UC Santa Cruz. She is focusing her energy on one element of the exceptional biodiversity here: a native fruit called caimito, or star apple. Parker slowly scoops the pulp out of one caimito and examines each seed carefully. They contain clues about the evolution of this fruit and how the people of Panama have shaped its history.

Her latest research involves both how edible fruits evolve in the tropics and how humans influence their ability to survive and reproduce. Little is known about how communities in the tropics have domesticated many formerly wild trees and fruits. The caimito is an excellent model to study such evolution, Parker says, because these fruits exist both in their primal, wild forms in the jungle and in cultivated forms in Panamanian backyards. “There is not a guessing game about what caimito fruits looked like before domestication took place,” Parker says—a much clearer picture than the blurry histories of most other edible fruits and plants.

Through her extensive work in the field, Parker has discovered that caimito fruits are native to Panama and were not imported from elsewhere. She has also unearthed that the people of Panama have been selecting favorable traits and planting these seeds in order to replicate larger, sweeter star apples—a trend evident in other fruits that have crossed over from wild populations to the marketplace.

Owning a house in Panama with her husband Greg Gilbert, a fellow biologist at UCSC, allows Parker to submerge herself into her research for two months each year. Though working in this habitat is a privilege, she does have to jump over difficult hurdles. For example, “seed ticks” the size of poppyseeds are the real struggle, Parker explains. Once while walking along, she stepped into a tick bomb—a nest of thousands of baby ticks— that spread quickly over her clothes. She then had to sprint back to her car, strip off her clothes, drive nearly naked back to her house, and soak her tick infested apparel in boiling hot water. “That was fun,” Parker says sarcastically as she laughs it off.

Parker first traveled to Panama during college to visit a friend. The visit quickly drew her back to research in the tropics. Beginning shortly after graduate school, her primary research focus revolved around tropical invasive plant species. It’s quite rare to find invasive plants in such a primal forest as Panama’s, according to Parker.

Her last visit took an unexpected turn when she began to examine the star apple. At first glance, she thought caimito trees were invasive species. Indeed, the scientific literature claimed they were introduced at some time to central Panama from other parts of the world. But when Parker dug deeper into the star apple’s history, she began to question its invasive reputation.

The star apple has a grand evolutionary history and cultural ethnobotany. “This plant is very widespread, and anyone over the age of 50 in Panama knows this fruit and has one in their garden,” Parker says. However, younger Panamanians don’t know of caimito because it is not sold in markets, due to the fruit’s short shelf life.

There’s also a great disconnect between what natural historians knew about the fruit and what the published literature maintained, says Parker. Ecologists have stated that this species originated in the Greater Antilles (near the Dominican Republic), and then people later brought it to Panama. In contrast, Panamanians insisted that the caimito trees were native to their land. People of Mexico also thought that caimito originated in Mexico. Parker was jolted by this discrepancy, so she set out to trace the evolutionary path of caimito fruits and how people may have changed its path by cultivating it.

Working with molecular phylogeneticists Jennifer Petersen and Dan Potter of UC Davis, Parker searched for clues about the origin of caimito fruits. They developed molecular markers to study where this fruit first arose by making phylogenetic trees of the relationships between caimito and its close relatives.  “It is like this historical puzzle we are trying to put together,” says Parker.

Scientists often use molecular markers in these genetic trees to examine how populations are connected to each other. A specific sequence of DNA is identified through the help of the known molecular markers, which reveal whether a particular sequence of DNA is neutral, or not under selection. The group also examined samples of caimito from other countries throughout North, Central, and South America, the Caribbean, and Asia, to find patterns of where the species arose, how it has become domesticated, and how exporting the fruit has allowed it to take root elsewhere.

In central Panama, the caimito shows a lot of genetic diversity. That’s a hallmark trait of where the fruit originated, Parker says. As they spread further away from Panama, the caimitos branch off into less varied groups. “This creates a pathway from which you can track how the fruit moved,” says Parker. From both tracking this path and the molecular data, the team determined that caimito fruits did in fact first appear in Panama—or somewhere very close by. The researchers still do not know exactly how old the star apple is, but “we know that it was present and already showing domestication traits [large fruits] at the time of Spanish contact” in the 16th century, Parker says.

Parker then examined the differences between the star apples that people grow in their gardens to those found in wild populations. “The cultivated and domesticated fruits look and act very differently than the wild ones,” states Parker. The wild greenish, pinkish fruit of caimito is about the size of a golf ball, while the domesticated fruits are deep purple and about the size of an orange. Parker found that domesticated fruits contain more pulp and sugar than wild ones. The cultivated fruits also have larger seeds with fewer defenses against disease. Wild caimito fruits are tougher and have fewer seeds. They also have a much more acidic pulp than the cultivated fruits.

By growing the largest, sweetest, and most colorful fruits in their backyards, the people of Panama have been changing the caimito gene pool without even knowing it. Just as with any species that we eat, at some point in the distant past, humans started selecting traits they liked in these species and planting the seeds of   the varieties that they wanted to replicate, Parker explains.

Caimito is only one example of a fruit that has been “genetically modified” for consumption. But with the caimito, scientists can study purely wild and cultivated fruits side by side—a rare occurrence. “What is different between caimito and a fully domesticated species like wheat is that with wheat we are only looking at the final product after thousands of years of intensive domestication,” says Parker.

Parker emphasizes that caimito fruits show us how diet has not only become homogenized in the tropics, but across the world. Each region and culture used to have a more specific diet. Now, any individual in any part of the world consumes a larger variety of food. “Now people in Panama go to supermarkets just like we do and buy apples from Washington, strawberries from Watsonville, and grapes from Chile,” Parker explains. Instead of growing more caimito and local fruits in their backyards, the people of Panama eat Washington apples.

Parker’s latest work involves doing a series of transplantation experiments in Panama. She plans on studying how semi-domesticated fruits react once introduced back into wild settings. She aims to understand how these “genetically modified” fruits will affect future wild caimito populations.

Parker explains that the evolutionary path of caimito fruits and how humans have changed their gene pool will help us understand the overall process of plant domestication. Most importantly, her studies of the star apple are not supposed to discourage Panamanians from growing these fruits in their backyards.

“I believe that caimito is a part of tropical fruit diversity and has a rich history with the people from the tropical Americas,” says Parker. “Under globalization, we are losing this diversity and the cultural diversity that goes with it.  Planting local fruits is an aspect of global food safety. At the same time, I think it is also important to preserve wild areas of tropical forest in Panama, to help preserve the genetic diversity of caimito in its region of origin.”

Kate Galloway, a junior majoring in marine biology, wrote this article in spring 2011 for SCIC 160: Introduction to Science Writing.

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